1. The Sun
6.E.1.2 Explain why Earth sustains life while other planets do not based on their properties (including types of surface, atmosphere and gravitational force) and location to the Sun.

2. The Sun’s Interior
Unlike Earth, the sun does not have a solid surface. Rather, the sun is a ball of glowing gas through and through. About three fourths of the sun’s mass is hydrogen and one fourth is helium. There are also small amounts of other elements. Like Earth, the sun has an interior and an atmosphere. The sun’s interior consists of the core, the radiation zone, and the convection zone.

3. The Core
The sun produces an enormous amount of energy in its core, or central region. This energy is not produced by burning fuel. Rather, the sun’s energy comes from nuclear fusion. In the process of nuclear fusion, hydrogen atoms join together to form helium. Nuclear fusion occurs only under conditions of extremely high temperature and pressure. The temperature inside the sun’s core reaches about 15 million degrees Celsius, high enough for nuclear fusion to take place.

4. The total mass of the helium produced by nuclear fusion is slightly less than the total mass of the hydrogen that goes into it. What happens to this mass? It is changed into energy. This energy slowly moves outward from the core, eventually escaping into space.

5. The Radiation Zone
The energy produced in the sun’s core moves outward through the middle layer of the sun’s interior, the radiation zone. The radiation zone is a region of very tightly packed gas where energy is transferred mainly in the form of electromagnetic radiation. Because the radiation zone is so dense, energy can take more than 100,000 years to move through it.

6. The Convection Zone
The convection zone is the outermost layer of the sun’s interior. Hot gases rise from the bottom of the convection zone and gradually cool as they approach the top. Cooler gases sink, forming loops of gas that move energy toward the sun’s surface.

7. The Sun’s Atmosphere
The sun’s atmosphere includes the photosphere, the chromosphere, and the corona. Each layer has unique properties.

8. The Photosphere
The inner layer of the sun’s atmosphere is called the photosphere (foh tuh sfeer). The Greek word photos means “light,” so photosphere means the sphere that gives off visible light. The sun does not have a solid surface, but the gases of the photosphere are thick enough to be visible. When you look at an image of the sun, you are looking at the photosphere. It is considered to be the sun’s surface layer.

9. The Chromosphere
During a total solar eclipse, the moon blocks light from the photosphere. The photosphere no longer produces the glare that keeps you from seeing the sun’s faint, outer layers. At the start and end of a total eclipse, a reddish glow is visible just around the photosphere. This glow comes from the middle layer of the sun’s atmosphere, the chromosphere (kroh muh sfeer). The Greek word chroma means “color,” so the chromosphere is the “color sphere.”

10. The Corona
During a total solar eclipse an even fainter layer of the sun becomes visible. This outer layer, which looks like a white halo around the sun, is called the corona, which means “crown” in Latin. The corona extends into space for millions of kilometers. It gradually thins into streams of electrically charged particles called the solar wind.

11. Features on the Sun
For hundreds of years, scientists have used telescopes to study the sun. They have spotted a variety of features on the sun’s surface. Features on or just above the sun’s surface include sunspots, prominences, and solar flares.

12. Sunspots
Early observers noticed dark spots on the sun’s surface. These became known as sunspots. Sunspots look small. But in fact, they can be larger than Earth. Sunspots are areas of gas on the sun’s surface that are cooler than the gases around them. Cooler gases don’t give off as much light as hotter gases, which is why sunspots look darker than the rest of the photosphere. Sunspots seem to move across the sun’s surface, showing that the sun rotates on its axis, just as Earth does. The number of sunspots on the sun varies over a period of about 11 years.

13. Prominences
Sunspots usually occur in groups. Huge, reddish loops of gas called prominences often link different parts of sunspot regions. When a group of sunspots is near the edge of the sun as seen from Earth, these loops can be seen extending over the edge of the sun.

14. Solar Flares
Sometimes the loops in sunspot regions suddenly connect, releasing large amounts of magnetic energy. The energy heats gas on the sun to millions of degrees Celsius, causing the gas to erupt into space. These eruptions are called solar flares.

15. Solar Wind
Solar flares can greatly increase the solar wind from the corona, resulting in an increase in the number of particles reaching Earth’s upper atmosphere. Normally, Earth’s atmosphere and magnetic field block these particles. However, near the North and South poles, the particles can enter Earth’s atmosphere, where they create powerful electric currents that cause gas molecules in the atmosphere to glow. The result is rippling sheets of light in the sky called auroras.

16. Solar wind particles can also affect Earth’s magnetic field, causing magnetic storms. Magnetic storms sometimes disrupt radio, telephone, and television signals. Magnetic storms can also cause electrical power problems.

17. Questions
The sun produces energy by A. attracting it with the force of gravity. B. nuclear fission. C. burning fuels such as oil. D. nuclear fusion.

18. What layer are you looking at when you look at an image of the sun. A
What layer are you looking at when you look at an image of the sun? A. photosphere B. chromosphere C. corona D. prominence

19. The solar wind is a stream of electrically charged particles that extend outward from the sun’s A. chromosphere. B. photosphere. C. corona. D. core.

20. The End!